1. Technical Field of Invention
The present invention relates generally to alkylation reactions. More particularly, the present invention relates to a method of improving alkylate yield in an alkylation reaction process having dual reactor schemes by exchanging the reactor contents between reaction and catalyst regeneration steps.
2. Description of Prior Art
Alkylation is a chemical process by which an alkylatable compound, such as isobutane or another similar branched saturated hydrocarbon (i.e. isoparaffin), is reacted with an alkylation agent, such as a low molecular weight olefin, in the presence of an acid catalyst to produce a higher molecular weight product. The product is an alkylate composed of a mixture of high-octane, branched-chain paraffinic hydrocarbons. Alkylate is a highly desirable gasoline blend stock because it is clean burning and has exceptional anti-knock properties.
It is known in the prior art to utilize a cyclic reactor scheme as a method of continuous operation of an acid catalyst alkylation process. A cyclic reactor scheme includes two or more reactors alternating between an alkylation reaction and catalyst regeneration. These alternating reactor schemes utilize solid acid catalysts, such as zeolite-containing catalysts. Solid acid catalysts are preferable to liquid acid (HF or H2SO4) catalysts because the latter are highly corrosive and can be toxic if accidentally released into the environment, such as via formation of gaseous hydrogen fluoride aerosols.
U.S. Pat. No. 5,986,158 to Van Broekhoven et al. illustrates a prior art alkylation process utilizing a cyclic reactor scheme. The alkylation process utilizes at least two catalytic reactors which cycle between an alkylation reaction mode and a catalyst regeneration mode. The catalyst utilized in the reactors is a solid catalytic material. The catalyst is known to promote the alkylation of isobutane with light olefin organic compounds (C3-C5) to produce alkylate having predominantly C7-C9 branched alkanes. Upon completion of the cycle times for the respective reaction and regeneration modes, the pair of reactors switch cycles, such that the regenerating reactor ceases regeneration mode and begins reaction mode and the reaction reactor ceases reaction mode and begins regeneration mode.
The Van Broekhoven patent does not, however, address the problem of a significant loss of alkylate product yield resulting from the co-mingling and reaction of alkylating reactant, such as C3-C5 olefins, with catalyst regenerent such as hydrogen. This co-mingling and reaction occurs at the beginning of each cycle when the reactors switch from the reaction mode to the regeneration mode and vice versa. The reactor entering its alkylation reaction step has just completed the catalyst regeneration step and still contains residual hydrogen from regeneration. As this reactor enters the alkylation reaction step, olefin addition begins and the olefins co-mingle and react with the residual hydrogen, resulting in the saturation of the olefin to its paraffinic form, for example, converting butene to butane. This lost saturated olefin is, therefore, unavailable for the alkyation reaction, resulting in the reduced yield of desired alkylate product. Similarly, a reactor entering the catalyst regeneration step still contains residual olefin from the just completed alkylation step. The olefin then mixes and reacts with the hydrogen injected during the ensuing catalyst regeneration.
These residual reactor contents for both reactors could be, for example, removed and purged to flare for disposal, transported to another holding vessel for later reprocessing, or delivered to downstream facilities where individual components can be separated, recovered, or reprocessed as appropriate. These alternatives, however, are inefficient and costly.
The prior art also includes other processes outside the field of refinery alkylation that operate cyclically and utilize a purge/displacement mode between operating steps. For example, one such process employs catalytic reactors that cycle between a reactor step, where light paraffinic hydrocarbons, such as propane and isobutene, are dehydrogenated to olefins, such as propylene and isobutene respectively, and a catalyst regeneration step, where catalyst activity is restored via the burn-off of coke residues with oxygen. Reactor purging is undertaken between these steps to prevent the undesirable reaction of hydrocarbons from the reactor step with oxygen from the regeneration step, which would result both in a product yield loss and potentially unstable reactor operating conditions caused by the resulting heat of combustion. Specifically, at the end of the reaction step and prior to the commencement of regeneration, steam is injected into the reactor to purge the contained hydrocarbons to the downstream product separation and recovery system. Similarly, at the end of catalyst regeneration step and prior to the commencement of the reaction step, hydrogen is injected into the reactors to react with the residual oxygen to form water and purge the reactors. The effluents from these purge/reaction steps are each directed to downstream processing facilities. There is no exchange of purged fluids between reactors. This results in a loss of materials and the creation of a waste stream. This method suffers from low efficiency and high cost and has not been utilized in alkylation processes.
Another process known in the art is a cyclic adsorption process that utilizes solid adsorbents such as molecular sieves for stream separation and purification. This is combined with an intermediate purge/displacement step, wherein the fluid contents of adsorbent vessels are exchanged between the cyclic steps of adsorption and desorption. Examples include processes utilized for normal paraffin/iso-paraffin separation and hydrogen purification. In the adsorption processes, however, there are no catalytic reactors or chemical reactions involved, but rather a separation of feed components, and the primary purpose of the purge/displacement exchange is to improve the separation factor and resulting purity of the recovered product(s).
A need exists for an alkylation method using a cyclic reactor scheme that results in a significant increase in the yield of alkylate product while maintaining its efficiency. A need also exists for an alkylation method that is economically and commercially viable and that minimizes capital costs associated with additional equipment.